1. Field of the Invention
The present invention relates to a fixing device and an image forming apparatus.
2. Description of Related Art
A typical image forming apparatus includes a fixing device that conveys a recording medium, such as paper, while fixing an image on the recording medium by pressure and heat applied to the recording medium.
In such an image forming apparatus, the recording medium conveyed by the fixing device may be jammed as foldings downstream in the conveying direction (for example, foldings S illustrated in FIG. 14). In such a case, the jammed recording medium may come into contact with rollers and belts in the fixing device and may damage the rollers and belts by applying pressure.
To detect such jamming, some image forming apparatuses include detection mechanisms disposed upstream and downstream of the fixing device in the conveying direction of the recording medium that detect the pass of the recording medium. In such an image forming apparatus, if a recording medium that has been detected to pass through the upstream area is not detected in the downstream area after a certain time, it is determined that the recording medium has been jammed in the downstream area, and operation is stopped.
However, with such a mechanism of detecting a jammed recording medium, the jammed recording medium can only be detected after a predetermined time. Thus, in some cases, the fixing device is damaged by the jammed recording medium before the operation of the image forming apparatus is stopped.
Nowadays, there is a need for image forming apparatuses capable of high-speed printing. An increase in printing speed requires an increase in the conveying speed of a recording medium conveyed by the fixing device. Thus, the rollers and belts of the fixing device are damaged by the foldings of a jammed recording medium in a shorter time once the recording medium is jammed. To prevent such damage of the fixing device, jamming of a recording medium must be detected at an earlier timing.
Accordingly, Japanese Unexamined Patent Application Publication No. 2003-57892 discloses an image forming apparatus including a lever that detects the pass of a recording medium by coming into contact with the recording medium in a conveying path downstream of a fixing device, and detecting jamming of a recording medium on the basis of the angle of the lever.
Unfortunately, the image forming apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2003-57892 causes a decrease in image quality due to scratches formed by the lever coming into contact with the image surface of the recording medium that has been fixed by the fixing device, regardless of jamming.
If the lever is disposed away from the conveying path such that the lever and the recording medium come into contact with each other only if the recording medium is jammed, the jammed recording medium cannot be detected in the early stage because the jamming has proceed to form foldings large enough to push up the lever to an angle that allows the jamming to be detected.
If the lever is simply disposed away from the conveying path, the recording medium and the lever may come into contact because the path the recording medium is conveyed through fluctuates. In such a case, the image forming apparatus erroneously detects a jamming and stops operation. Hence, productivity of the image forming apparatus is reduced due to such erroneous detection.